1. Field of the Invention
The present invention relates to a method for mounting an electronic component in which an adhesive layer having anisotropic conductivity is provided between a circuit pattern of a circuit board and electrodes of an electronic component.
2. Description of the Related Art
Conventionally, as one of technologies for joining an electronic component to a circuit pattern of a circuit board, a method has been carried out wherein an electronic component and a circuit board facing the component are thermally bonded with an anisotropic conductive material composed of an adhesive layer in which conductive particles are dispersed, and a conductive connection between a circuit wiring and the electrodes of the electronic component is formed as disclosed in Japanese Unexamined Patent Kokai Publication No. 8-138773. However, the method disclosed in the above Japanese Unexamined Patent Kokai Publication could not be used for the purpose of connecting a wiring board and an electronic component having insufficient heat resistance or connecting a member and a part having insufficient thermal-dimensional stability.
Then, a method for connecting an electronic component to a circuit board without heating it by use of an anisotropic conductive pressure-sensitive adhesive sheet material having stickiness has been developed. The method comprises, for example, sticking an anisotropic conductive post-photocurable type pressure-sensitive adhesive sheet material having stickiness on a board surface on which an electronic component is to be mounted; irradiating the adhesive sheet material with light to activate the curing reaction of the adhesive sheet material, followed by placing and pressing (or attaching) the electronic component onto the adhesive sheet material while the adhesive sheet material still keeps the stickiness; and promoting the curing reaction of the adhesive sheet material to securely bond them together, as disclosed in Japanese Unexamined Patent Kokai Publication No. 11-256117. Relating to the present invention, the stickiness may be referred to as tackiness or (pressure sensitive) adhesiveness. In accordance with the above method, it takes a considerable length of time until a desired mounting board is obtained, because it is necessary to cure the adhesive sheet material at a room temperature for a predetermined length of time until the curing reaction is completed after light irradiation,
Further, when such a photocurable type anisotropic conductive pressure-sensitive adhesive sheet material is used, it is necessary to test whether or not an electrical connection has been actually formed between an electronic component and a circuit board with the pressure-sensitive adhesive sheet material in some cases. Such an inspection is to be carried out after the curing reaction requiring a predetermined time passage is completed. Thus, when a defect would be found in the inspection, the anisotropic conductive pressure-sensitive adhesive sheet material has been already cured, which makes it impossible to detach and join the electronic component again without breaking the board and the surface of the electronic component. Therefore, circuit boards which have been rejected in the inspection have been handled as defective products directly. Accordingly, it is impossible to readjust an electronic component attached to a circuit board at the stage of inspection, and the generation of some defective products has been one factor that causes a reduction in productivity.
Moreover, it can be expressed that, during the time immediately after light irradiation up to completion of a curing reaction, the electronic component attached onto the circuit board is not fixed onto the circuit board virtually, but is merely placed thereon for a relatively long time. Accordingly, after light irradiation, it is necessary to provide a place where the circuit board on which the electronic component being attached is temporarily stored or the curing reaction of the circuit board is completed and also a time period therefor. When a place where the circuit board is temporarily stored is located away from the place where light irradiation is carried out and/or where the electronic component is placed, it is further necessary to move the circuit boards to the located place by a carrying device such as a belt conveyer. At the time of conveying it, if the attached electronic component would had relatively large mass, and if a direction and a level of acceleration applied to the electronic component during conveying the circuit boards would vary discontinuously, problems involving that the electronic component attached onto the circuit board is displaced on the circuit board due to inertial force applied to the electronic component tend to occur. The displacement of an electronic component on a circuit board has been a great factor of a poor connection.
In Japanese Unexamined Patent Kokai Publication No. 2000-86987, an invention of a joining method which is capable of rejoining an electronic component joined onto a circuit board. This method is the same as the invention in the aforementioned Japanese Unexamined Patent Kokai Publication No. 11-256117, in that an anisotropic conductive photocurable type pressure-sensitive adhesive sheet material having stickiness is stuck on a board or an electronic component, and the adhesive sheet material is irradiated with light, and after a curing reaction is activated, the circuit board and the electronic component are securely bond together while the adhesive sheet material still keeps the stickiness. This method makes it possible to detach electronic components from the circuit board without breaking the surface of the board and the electronic component and rejoin the electronic component onto the circuit board by using a material comprising an acrylic polymer, a cationic polymerization compound, a photo cationic polymerization initiator, and conductive particles as a material for the pressure sensitive adhesive layer. However, the generation of the defect caused at the time of joining and the defect caused by conveying the board after light irradiation has not been sufficiently prevented according to the method disclosed in Japanese Unexamined Patent Kokai Publication No. 2000-86987.